"Whet" is to sharpen a blade, so it means figuratively to make appetite keener.

Which brings me to a genuine peeve:"Hone" is also to sharpen a blade, or metaphorically a skill or interest -- so where do we get "hone in on" something, tracking something and getting closer to it, when the phrase should be "home in on" the thing? This is the problem again of learning mostly by (mis)hearing and not doing enough reading to see as well as hear an idiom or other usage.

These people are all correct. This one is intuitive, and the problem only arises when people get "whet your appetite" confused with "wet your whistle", making these phrases only a bit less confusing than the "cut the mustard"/"pass muster" problem:

"*Wet your whistle' predates 'whet your appetite' by some centuries, and was first recorded in the 1386 Towneley Mysteries:

There are two expressions in question there, Dotter. To "home in" on something would be to gain a more accurate fix on its location by radio signal. To "horn in" on something (or someone) is to impose oneself in a way that interferes or intrudes -- to horn in on a conversation, for example. The expression that is incorrect is to "hone in," which is often misused in place of "home in" by people who do not visualize the expression and are not familiar with its original meaning. "Hone in" makes no sense when analyzed -- to hone is to sharpen.

In response to chironsdaughter's post.... Just want to point out that sometimes a technical meaning is more useful and definitive than a dictionary description. To hone a blade is not to sharpen it; rather it is to reshape the burr of the blade (which develops over time) into a more effective shape. To sharpen a blade is to remove steel from it and then polish the edge to the desired shape and bevel. Whetstones are used to remove steel from a blade and therefore sharpen, while a ceramic honing steel or a strop block are to perfect/polish/upkeep the shape rather than actually change it.

Whet is useful from my experience as a long-time cook for one of two applications; to whet ones palate or appetite by utilizing appropriate dishes and/or drinks in a calculated succession, or as in a whetstone, which is solely used for the purpose of changing the blade, (and hopefully sharpening it if you do it correctly). I've not experienced any other applications for the word.

As Iron Sharpens Iron - an iron(steel) will bring a Sword(the Word[or your words{that is your thinking and ability to Reason} to a better edge when it is properly whetted against it. So one person or 'friend', can, through discussion with you, cause you, to reflect and meditate upon an issue from another Point-of-view. This meditation can have it's effect on you and may lead you, like a Horse To Water, to See Things in a different Light. So Whetting Your Appetite(for some thing) Much like a salesman whets the appetite of the Mark, or a Brother whets the appetite of a Neophyte for the Word. And so through Brotherly Love(castigation[or castration of neophyte on his journey to (eunuch) priesthood]) your Sword(oration) can too become Sharpened(cleverer) too as you Seek the Light brother.